Selecting receiver chains of a mobile unit for receiving wireless signals

ABSTRACT

A method and apparatus of selecting which of a plurality of receiver chains of a mobile unit to receive wireless signals, is disclosed. One method includes measuring a first receive signal quality while all of the plurality of receiver chains are receiving wireless signals, and measuring a second receive signal quality while a subset of the plurality of receiver chains are receiving wireless signals. The subset of the plurality of receiver chains are selected to receive wireless signal unless the first receive signal quality is a threshold better than the second receive signal quality. If the first receive signal quality is a threshold better than the second receive signal quality then all the plurality of receiver chains are selected to receive wireless signals.

FIELD OF THE DESCRIBED EMBODIMENTS

The described embodiments relate generally to wireless communications.More particularly, the described embodiments relate to methods andsystems for selecting receiver chains of a mobile unit for receivingwireless signals.

BACKGROUND

Wireless devices can include fixed wireless devices or mobile wirelessdevices. Fixed wireless devices can include base stations and/or fixedlocation consumer-used devices. Fixed wireless devices typically haveessentially unlimited power available since there are typically poweredby an AC power source. Mobile wireless devices, however, are typicallybattery powered, and therefore, must conserve power usage in order toensure a reasonable time of operation.

New wireless systems and standards, such as 802.16e or WiMAX aredeploying devices with multiple antenna devices that can incorporatesmart antenna technologies to improve the quality of wireless linksbetween the wireless fixed and/or mobile devices. Multiple antennadevice, however, typically dissipate more power than single antennadevices because multiple antenna devices generally require a receiverchain (amplifiers, local oscillators, and frequency mixers) for each ofthe antennas of the multiple antenna device.

Therefore, a conflict exits between the desire for less powerdissipation of mobile units, and the desire to improve communicationlinks of mobile unit through the use of multiple antennas.

It is desirable to have a system and method for reducing powerconsumption of multiple antenna mobile units of a wireless system whilemaintaining a desired level of performance.

SUMMARY

An embodiment includes a method of selecting which of a plurality ofreceiver chains of a mobile unit to receive wireless signals. The methodincludes measuring a first receive signal quality while all of theplurality of receiver chains are receiving wireless signals, andmeasuring a second receive signal quality while a subset of theplurality of receiver chains are receiving wireless signals. The subsetof the plurality of receiver chains are selected to receive wirelesssignal unless the first receive signal quality is a threshold betterthan the second receive signal quality. If the first receive signalquality is a threshold better than the second receive signal qualitythen all the plurality of receiver chains are selected to receivewireless signals.

Another embodiment includes a method of selecting which of a pluralityof receiver chains of a mobile unit to receive wireless signals. Themethod includes measuring a first receive signal quality while N of theplurality of receiver chains are receiving wireless signals. A secondreceive signal quality is measured while M of the plurality of receiverchains are receiving wireless signals. M receiver chains are selected toreceive wireless signal unless the first receive signal quality is athreshold better than the second receive signal quality. However, if thefirst receive signal quality is a threshold better than the secondreceive signal quality then N of receiver chains are selected to receivewireless signals, wherein N>M.

Another embodiment includes a method of selecting which of a pluralityof receiver chains of a mobile unit to receive wireless signals. Themethod includes the mobile unit sensing a mobile unit condition notrelated to reception of wireless signals. The subset of the plurality ofreceiver chains is selected to receive wireless signal if the mobileunit condition exceeds a threshold. If the threshold condition fallsbelow the threshold then all the plurality of receiver chains areselected to receive wireless signals. The mobile unit condition caninclude a level of battery power available, or a temperature level ofthe mobile unit.

Other aspects and advantages of the described embodiments will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a wireless network that includes a basestation that is wirelessly communicating with multiple antenna mobileunits.

FIG. 2 shows an example of multiple receiver chains of a multipleantenna mobile unit.

FIG. 3 is a flow chart that includes steps of one example of a method ofselecting which of a plurality of receiver chains of a mobile unit toreceive wireless signals.

FIG. 4 shows a WiMAX frame that communicates information between a basestation and mobile units, and includes a preamble.

FIG. 5 is a flow chart that includes steps of one other example of amethod of selecting receiver chains of a mobile unit to receive wirelesssignals.

FIG. 6 is a flow chart that includes steps of one other example of amethod of selecting receiver chains of a mobile unit to receive wirelesssignals.

DETAILED DESCRIPTION

Embodiments of methods of selecting which of a plurality of receiverchains of a mobile unit to receive wireless signals are disclosed. Thenumber of receiver chains that are selected is dependent upon detectinga threshold difference in the quality of signals received by differentnumbers of receiver chains. Various operational parameters or signalquality characteristics of either the mobile unit or a transmitting basestation can influence the value of the threshold.

FIG. 1 shows an example of a wireless network that includes a basestation 110 that is wirelessly communicating multiple antenna mobileunits 120, 122, 124. As shown, the base station 110 transmits wirelesssignals that can be received by mobile units 131, 132, 133, 134, 135located within a transmission range or cell as shown.

If a mobile unit is located within the transmission range or cell,generally the mobile unit can successfully receive wireless signals fromthe base station 110. However, the signal quality of the receive signalcan vary depending on the location of the mobile unit within thetransmission range or cell. More specifically, the transmission signalof the base station typically is attenuated as the transmission signalpropagates away from the base station. Therefore, the amplitude of thereceived transmission signal is typically much greater when the mobileunit is proximate to the base station 110 than when the mobile unit islocated near the edge of the transmission range or cell.

For example, the mobile units 132, 135 are located near the base station110, and therefore, probably receive signals from the base station 110having a relatively high signal level. The relatively high signal leveltypically ensures the received signals have a relatively high CINR(carrier to interference plus noise ratio). However, the mobile unit 133is located near the outer edge of the cell of the base station 110, andtherefore, probably receives signals from the base station 110 having arelatively low signal level and CINR.

Mobile units receiving signals having relatively higher CINRs cansupport higher data rates than mobile units receiving signals havingrelatively low CINRs. Therefore, based on receive signal CINRs, the datarates that can be supported by the mobile units vary depending uponwhere the mobile unit is located with respect to the base station.

It has been well established that the use of multiple or additionalantennas and receiver chains can be used to improve the data rates ofreceive signals. That is, for example, smart antenna techniques, suchas, receive diversity and spatial multiplexing can be utilized bymultiple antenna receivers to improve the data rates of wireless signaltransmitted to the multiple antenna receivers. Therefore, smart multipleantenna techniques can be used to improve, for example, the data ratesof mobile units that are located farther away from a base station.

As previously described, however, multiple antenna receivers typicallyrequire a receiver chain for each receive antenna. FIG. 2 shows anexample of multiple receiver chains of a multiple antenna mobile unit.Each of the receiver chains 210, 220, 230 includes at least a couple ofamplifiers 212, 214, 222, 224, 232, 234 and a frequency translator 214,224, 234. Typically, each receive chain includes a band pass filterafter the first amplifier (for example, amplifiers 212, 222, 232), and alow pass filter after the frequency translator. Clearly, the morereceiver chains that are activated by the mobile unit for reception ofwireless signals, the greater the power dissipated by the mobile unit topower the active devices of the receiver chains.

FIG. 3 is a flow chart that includes steps of one example of a method ofselecting which of a plurality of receiver chains of a mobile unit toreceive wireless signals. A first step 310 includes measuring a firstreceive signal quality while all of the plurality of receiver chains arereceiving wireless signals. A second step 320 includes measuring asecond receive signal quality while a subset of the plurality ofreceiver chains are receiving wireless signals. A third step 330includes selecting the subset of the plurality of receiver chains toreceive wireless signal unless the first receive signal quality is athreshold better than the second receive signal quality, if the firstreceive signal quality is a threshold better than the second receivesignal quality then selecting all the plurality of receiver chains toreceive wireless signals.

Different methods and measurements can be used to determine whether thefirst receive signal quality is a threshold better than the secondreceive signal quality. Additionally, the threshold can be adjustedbased on at least one of several different factors.

One embodiment includes determining a first modulation and coding formatthat the first receive signal quality enables the use of, anddetermining a second modulation and coding format that the secondreceive signal quality enables the use of. The first receive signalquality can be determined to be the threshold better than the secondreceive quality if a data throughput supported by the first modulationand coding format is a predetermined percentage greater than a datathroughput supported by the second modulation and coding format.Basically, the use of less than all the receiver chains are selected toreceive the wireless signals, unless receiving wireless signals throughall of the receiver chains provides a data throughput that is at leastthe threshold better than the data throughput provided by receiving thewireless signals through less than all of the receiver chains.

Another embodiment includes selections between N receiver chains and Mreceiver chains rather than just between all receiver chains and asubset of receiver chains. However, the same basic selection conditionsapply. That is, for example, a first signal quality measured whilereceiving through a first number of receiver chains is compared with asecond signal quality measured while receiving through a second numberof receiver chains. Comparing a difference between the first signalquality and the second signal quality with a predetermined threshold isused to determine the selection of either the N receiver chains or the Mreceiver chains, wherein N is greater than M. As described, thethreshold can be adaptively adjusted based on at least one of severaldifferent parameters and/or conditions. One embodiment includes agraduated set of thresholds, in which the number of receiver chainsselected varies depending upon, for example, the signal qualitiesmeasured for each of the combinations of receiver chains. The number ofreceiver chains can be adaptively selected base on the receive signalqualities of the combinations, and the amount of battery power availableto the mobile unit.

Embodiments of the wireless signals include frames. More specifically,an embodiment of the frames of the wireless signals includes a preamblewithin each frame. The preamble includes known signal qualities andcharacteristics, such as, pilot tones and known data. A receiver canmeasure signal qualities of the received signal, and estimate thetransmission channel between the transmitter and the receiver bycomparing the measured signal qualities with the known characteristicsof the transmitted signal. The preamble portions of the frames provide adesirable time to measure the signal quality of the receive signals. Anembodiment includes the first receive signal quality and the secondreceive signal quality being measured during the preamble of receivedsignals, and the number of receiver chains for receiving the signalsselected.

The preambles of most wireless signals occur at the start of the frames.An advantage of performing the signal quality measurements during thepreamble as opposed to during another portion of the frame (such asduring control and/or data transmission) is that the benefits providedby the selection between the different number of receiver chains can berealized early in the frame. Therefore, the power dissipation versustransmission signal quality tradeoffs optimized for a greater amount oftime.

Another embodiment includes the first receive signal quality and thesecond receive signal quality being measured once every k frames.Clearly, this requires less overhead than is required if the signalqualities were to be measured every frame. One other embodiment includesthe value of k being adaptively selected based upon at least onecharacteristics of a transmission channel between the base station andthe mobile unit. Once embodiment includes the value of k beingadaptively selected based on a coherence time (Doppler shift) of thetransmission channel. The coherence time provides an indication of howfast the transmission channel is changing.

Various receive signal quality parameters can be used for establishing(measuring) the first and second receive signal qualities. Someexemplary signal qualities that can be measured include spectralefficiency, CINR, CNR, PER, BER, BLER, energy consumed per receive bit,MAP error rate, and absolute CINR of the first receive signal quality.

The spectral efficiency can be particularly useful because it providesan indication of the data rates that can be sustained by thetransmission signals between the transmitter and the differentcombinations of receiver chains of the multiple antenna receiver.Mathematically, the spectral efficiency of a MIMO (multiple input,multiple output) transmission signal can be estimated by:

spectral efficiency=log₂(det(I _(NR)+(ρ/NT)H H*));

where I is an identity matrix, NR is the number of active receiveantennas, NT is the number of active transmit antennas, ρ is the receivesignal CINR, H is an estimate of the channel between the transmitter andreceiver which has the dimension of NR×NT, and H* is the Hermitian of H.

Another embodiment includes determining the spectral efficiency byreferencing a look up table based on the measured CINR.

As previously described, the transmission channel can be estimated andthe CINR measured during the preamble of a frame of the wirelesstransmission signal. The spectral efficiency can then be determinedalong with knowledge of the number of transmitter and receiver antennas.

An embodiment further includes estimating the spectral efficiency duringthe preamble of a frame, and then augmenting the estimate withadditional knowledge obtained during the remaining duration of theframe. For example, additional signal quality information can beobtained during control signal and data transmission portions of theframe (more specifically, during the downlink portion of the frame). Theadditional signal quality information can augment or refine the spectralefficiency estimate. The possibility exists that the augmented spectralefficiency exceeds the predetermined threshold when the initial spectralefficiency did not, or the augmented spectral efficiency does not exceedthe predetermined threshold when the initial spectral efficiency did.

Another embodiment includes the initial spectral efficiency beingdetermined during single antenna transmission of the preamble of theframe, and the spectral efficiency being augmented during single antennatransmission of control information, and multiple antenna transmissionduring the data transmission portion of the frame. That is, the spectralefficiency is initially estimated with signal antenna transmissionsignals during the preamble of the frame, and the estimated spectralefficiency is augmented during single antenna and/or multiple antennatransmission later in the frame.

As previously described, the threshold can be adapted. For example, oneembodiment includes the threshold being dependent upon an estimatedamount of battery power available to the mobile unit. That is, forexample, the threshold can be dynamically adjusted as the availablebattery power of the mobile unit changes.

An embodiment includes comparing CINR of the first receive signalquality with the CINR of the second receive signal quality. Thethreshold can be dynamically adjusted based on the amount of remainingbattery life of the mobile unit. For example, if the remaining batterylife of the mobile unit is low, the threshold can be selected to berelatively low, so that only one antenna and receiver chain is selectedthe majority of the time. However, if the battery life is sensed to behigh, then the threshold can be set relatively high, and therefore, thelikelihood of selecting two or more antennas for receiving wirelesssignals is greater, and will happen a greater percentage of the time.

Another embodiment includes the threshold being dependent upon anestimation of energy consumed per bit for receiving wireless signalsthrough all of the plurality of receiver chains and an estimation ofenergy consumed per bit for receiving wireless signals through thesubset of the plurality of receiver chains. The energy consumed per bitcan be estimated by observing the modulation and coding for thedifferent combinations of antenna and receiver activations, and dividingby the energy being consumed for the different combinations.

Embodiments include the base station that transmits the wireless signalsinitiating and/or maintaining at least some control over how many of thereceive chains the mobile unit uses to receive wireless signals.Embodiments include the threshold being adaptively adjusted based uponcharacteristics of the base station that is transmitting the wirelesssignals. For example, one embodiment includes the threshold beingdependent upon the loading of a base station that is transmitting thewireless signals. The loading of the base station can be determined bythe number of mobile units (or more generally, users) being served orcommunicating with the base station, and by a level of interferenceobserved during the uplink frame from other users of other cells.

Other embodiments include the threshold be adaptively selected basedupon characteristics of the information being communicated by thewireless signals being received. For example, one embodiment includesthe threshold being dependent upon latency and throughput requirementsof the information of the received wireless signals. The latency andthroughput can associated with the downlink and/or uplink transmissionbetween the base station and the mobile unit. Some types of informationbeing communicated (such as, voice data) is sensitive to latency andthroughput of the uplink and/or downlink.

Other embodiments include the base station having at least some controlover the number of receive chains the mobile unit uses to receivewireless signals based on feedback information provided by the mobileunit to the base station. For example, embodiments include the mobileunit reports its remaining battery power to the base station. Aspreviously described, the receive chain activation can be based on, oradaptively adjusted based on the remaining battery power of the mobileunit. For this embodiment, the base station at least partially controlsthe threshold based on reported remaining battery power.

Embodiments include control of how the mobile unit reports informationback to the base station. For one embodiment, the mobile unit reportsthe remaining battery life through a control channel, wherein thecontrol channel includes a MAC layer packet sent by the mobile unit tothe base station. For another embodiment, the mobile unit reports theremaining battery life through a control channel, wherein the controlchannel includes an application layer packet of a server associated witha network of the base station. To reduce the impact of the reporting onavailable bandwidth, an embodiment includes mobile unit periodicallyreporting the remaining battery life to the base station through thecontrol channel.

An embodiment includes the signal qualities and/or channel qualitiesbeing reported from the mobile unit to the base station. Some wirelessstandards, such as, WiMAX include provisions for reporting channelqualities through a channel quality indicator. One specificimplementation includes the mobile unit reporting channel quality(CQICH) to the base station. For an embodiment, the reported channelquality is for signal quality for reception of through a signal receiverchain. The reported channel quality can be for a single receiver chaineven if multiple of the receiver chains are active.

Various embodiments address different ways the mobile unit receivesinstructions from the base station. One embodiment includes the mobileunit activating only the subset of receiver chains unless a transmittingbase station indicates that the transmitting base station has data totransmit to the mobile unit. Another embodiment includes the mobile unitreceiving a command from a transmitting base station that indicates tothe mobile unit how many receiver chains to activate, and for how manyframes of transmission of the wireless signals. That is, the mobile unitmaintains reception over the number of receiver chains as indicated bythe base station for a number of frames as indicated by the basestation.

Another embodiment includes the mobile unit maintaining the number ofreceiver chains until the base station indicates a change. This can beadvantageous because the base station transmits data to the mobile unitat the high data rate available with a greater number of receiverchains. The base station can then control when to reduce the number ofreceiver chains which support a lower data rate.

Another embodiment includes the mobile unit receiving a command from atransmitting base station. Upon receiving the command, the mobile unitperiodically selecting a preset multiple of receiver chains to receivewireless signals for a predetermined duration, over a predeterminedinterval.

Another embodiment includes the mobile unit detecting a handoffcondition. That is, the mobile unit determines that is should handoffwireless communication to a different base station for maintainingwireless communication. For this embodiment, if the mobile unit detectsa handoff condition, the mobile unit selects all the plurality ofreceiver chains to receive wireless signals. This is desirable because,for example, the CINR of receive signals tends to be low in a handoffregion. That is, the mobile is near an edge of a cell of the basestation, and therefore, probably receiving relatively lower amplitudesignal from the base station. The mobile unit can attempt to improve isreceive signal quality (CINR) be activating and receiving over a greaternumber of receive antenna and receiver chains.

Another embodiment includes the mobile unit reporting channel quality tothe base station, and the base station selecting all or a subset of thereceiver chains based on the reported channel quality. The mobile unitcan report the quality of one of the receiver chain settings (all or asubset), and the base station can infer what the quality would be forthe other setting. For example, the base station can in some situations,infer a 3 dB improvement for a two receiver chain selection versus a onereceiver chain selection. The base station can determine the receiverchain selection based on the reported channel quality and the inferredchannel quality.

FIG. 4 shows a WiMAX frame that communicates information between a basestation and mobile units. As shown, a downlink frame includes 33multiple carrier symbols, and an uplink frame includes 15 multiplecarrier symbols. Each multi-carrier symbol includes 1024 sub-carriers.

The downlink and uplink frames are divided into different functionportions. For example, the downlink frame includes a preamble, and FCH(frequency control header), a DL (downlink) MAP, a UL (uplink) MAP andDL data. The first symbol is the preamble which is typically used forsuch things as system acquisition, cell identification, handoff, timingand frequency tracking, and channel estimation. The symbols followingthe preamble contain control information about the rest of the frame.The symbols after the control data (FCH) is the downlink data symbols.

As previously described, an embodiment includes the mobile unitestimating spectral efficiency of received signals of differentcombinations receiver chains. One specific embodiment estimates aninitial spectral efficiency during the preamble, and then augments theinitial estimate during later portions (such as, the control informationof the FCH and MAP, and then the downlink data) of the frame. Thereceiver chain selection (that is, which receiver chains are used toreceive the wireless transmission signal) is made based on the initialspectral efficiency estimate of the preamble. The receive chainselection can change, however, during the frame as the spectralefficiency is augmented or refined over the duration of the frame. Thepreamble can be transmitted from a single antenna, whereas the downlinkdata can be transmitted from multiple antennas.

FIG. 5 is a flow chart that includes steps of one other example of amethod of selecting which of a plurality of receiver chains of a mobileunit to receive wireless signals. A first step 510 includes measuring afirst receive signal quality while N of the plurality of receiver chainsare receiving wireless signals. A second step 520 includes measuring asecond receive signal quality while M of the plurality of receiverchains are receiving wireless signals. A third step 530 includesselecting M receiver chains to receive wireless signal unless the firstreceive signal quality is a threshold better than the second receivesignal quality, if the first receive signal quality is a thresholdbetter than the second receive signal quality then selecting N ofreceiver chains to receive wireless signals.

FIG. 6 is a flow chart that includes steps of one other example of amethod of selecting which of a plurality of receiver chains of a mobileunit to receive wireless signals. A first step 610 includes the mobileunit sensing a mobile unit condition not related to reception ofwireless signals. A second step 620 includes selecting the subset of theplurality of receiver chains to receive wireless signal unless themobile unit condition exceeds a threshold. If the threshold conditionfalls below the threshold then selecting all the plurality of receiverchains to receive wireless signals. The mobile unit condition caninclude a level of battery power available, or a temperature level ofthe mobile unit. If the battery power available fails below a threshold,or the temperature level is above a threshold, the mobile unit canselect fewer receiver chains, and vice versa.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The invention islimited only by the appended claims.

1. A method of selecting which of a plurality of receiver chains of a mobile unit to receive wireless signals, comprising: measuring a first receive signal quality while all of the plurality of receiver chains are receiving wireless signals; measuring a second receive signal quality while a subset of the plurality of receiver chains are receiving wireless signals; selecting the subset of the plurality of receiver chains to receive wireless signal unless the first receive signal quality is a threshold better than the second receive signal quality, if the first receive signal quality is a threshold better than the second receive signal quality then selecting all the plurality of receiver chains to receive wireless signals.
 2. The method of claim 1, wherein determining whether the first receive signal quality is a threshold better than the second receive signal quality comprises: determining a first modulation and coding format that can be supported by the first receive signal quality; determining a second modulation and coding format that can be supported by the second receive signal quality; determining the first receive signal quality is the threshold better than the second receive quality if a data throughput supported by the first modulation and coding format is a predetermined percentage greater than a data throughput supported by the second modulation and coding format.
 3. The method of claim 1, wherein the threshold is dependent upon an estimated amount of battery power available to the mobile unit.
 4. The method of claim 1, wherein the received wireless signals comprise frames, each frame comprising a preamble, and the first receive signal quality and the second receive signal quality are measured during the preamble.
 5. The method of claim 4, further comprising refining the first and second signal qualities based on received symbols of the frame occurring after the preamble.
 6. The method of claim 4, wherein the first receive signal quality and the second receive signal quality are measured once every k frames.
 7. The method of claim 6, wherein the value of k is adaptively selected based on at least one channel characteristic.
 8. The method of claim 1, wherein the mobile unit measures the first receive signal quality and the second receive signal quality, and the first receive signal quality and the second receive signal quality comprises at least one of CINR, CNR spectral efficiency, PER, BER, BLER, energy consumed per receive bit, MAP error rate.
 9. The method of claim 1, wherein the threshold is dependent on loading of a base station that is transmitting wireless signals that are received by the mobile unit.
 10. The method of claim 1, wherein the threshold is dependent on CINR of the first receive signal quality.
 11. The method of claim 1, wherein the threshold is dependent upon latency and throughput requirements of information of the received wireless signals.
 12. The method of claim 1, wherein the threshold is dependent upon an estimation of energy consumed per bit for receiving wireless signals through all of the plurality of receiver chains and an estimation of energy consumed per bit for receiving wireless signals through the subset of the plurality of receiver chains.
 13. The method of claim 1, further comprising a base station selecting all the plurality of receiver chains or the subset of receiver chains at the mobile unit.
 14. The method of claim 1, wherein the mobile unit reports its remaining battery power to the base station.
 15. The method of claim 14, wherein the mobile unit reports the remaining battery life through a control channel, wherein the control channel comprises a MAC header packet of the base station.
 16. The method of claim 14, wherein the mobile unit reports the remaining battery life through a control channel, wherein the control channel comprises an application layer packet of a server associated with a network of the base station.
 17. The method of claim 14, wherein the mobile unit periodically reports the remaining battery life to the base station through the control channel.
 18. The method of claim 1, wherein the mobile unit activates only the subset of receiver chains unless a transmitting base station indicates that the transmitting base station has data to transmit to the mobile unit.
 19. The method of claim 1, further comprising the mobile unit receiving a command from a transmitting base station that indicates to the mobile unit how many receiver chains to activate, and for how many frames of transmission of the wireless signals.
 20. The method of claim 1, wherein if the mobile unit detects a handoff condition, the mobile unit selects all the plurality of receiver chains to receive wireless signals.
 21. The method of claim 1, further comprising the mobile unit indicating to a transmitting base station how many of the plurality of receiver chains are receiving wireless signals.
 22. The method of claim 1, further comprising the mobile unit reporting channel quality to the base station, and the base station selecting all or a subset of the receiver chains based on the reported channel quality.
 23. The method of claim 1, further comprising: the mobile unit receiving a command from a transmitting base station; upon receiving the command, the mobile unit periodically selecting a preset multiple of receiver chains to receive wireless signals for a predetermined durations, over a predetermined interval.
 24. A method of selecting which of a plurality of receiver chains of a mobile unit to receive wireless signals, comprising: the mobile unit sensing a mobile unit condition not related to reception of wireless signals; and selecting the subset of the plurality of receiver chains to receive wireless signal unless the mobile unit condition exceeds a threshold, if the threshold condition falls below the threshold then selecting all the plurality of receiver chains to receive wireless signals.
 25. The method of claim 24, wherein the mobile unit condition is at least one of mobile unit battery life depletion, mobile unit heat dissipation.
 26. A method of selecting which of a plurality of receiver chains of a mobile unit to receive wireless signals, comprising: measuring a first receive signal quality while N of the plurality of receiver chains are receiving wireless signals; measuring a second receive signal quality while M of the plurality of receiver chains are receiving wireless signals; selecting M receiver chains to receive wireless signal unless the first receive signal quality is a threshold better than the second receive signal quality; selecting N of receiver chains to receive wireless signals if the first receive signal quality is a threshold better than the second receive signal quality. 